This invention relates to phosphors and more particularly to phosphors having a light modifying layer thereon. These phosphors can be used, for example, on the image screen of cathode ray tubes, in particular the image screen of color cathode ray tubes and, when so used, they also increase the contrast of the tubes. Phosphors are known classes of materials which have the property of emitting radiation in the form of light when bombarded by a suitable form of energy such, for example, as the electron rays generated in a cathode ray tube.
In a color cathode ray tube, for example, the image screen is provided with a discrete pattern of phosphors emissive of the three primary colors: red, green, and blue. As used generally in the art and particularly in this specification, a phosphor referred to by a color will mean the light emitted by the phosphor as opposed to its body color.
While it would be advantageous if these phosphors were line emitters, typically they are not. For example, a red phosphor will typically emit radiation over a frequency range wherein the net overall effect to a subjective observer will be red. However, there will be extraneous unwanted emission points and frequencies within the spectrum which may or may not detract from the overall perception of the phosphor. For example, some red phosphors have emission peaks within the orange portion of the spectrum which greatly enhance the brightness characteristics of the phosphor but tend to detract from the so called "ideal" red color desired. Likewise, the blue and green phosphors emit over a frequency range containing desired and undesired frequencies.
The art has been aware of these problems for many years and frequent attempts have been made to obviate these problems. For example, it has been suggested that appropriate filters may be placed in front of the discrete phosphors to absorb or attenuate the undesired frequencies while passing the desired ones. This approach has proven to be very expensive and very difficult to achieve. More recently it has been suggested to apply a filter material to the phosphor particles themselves. This approach has been detailed recently in such U.S. Pats. as Nos. 3,875,449; 3,308,326; and 3,886,394. Each of these prior patents discloses various means for accomplishing the application of a filter material to the phosphor particle. However, each of them is accompanied by difficulties in application and in uniformity or in the agglomeration of the phosphor particles which makes them difficult to apply to the screen area of a cathode ray tube.
It would therefore be an advance in the art if means of obviating the problems of the prior art could be accomplished.